Turbo charged engines utilize a Charge Air Cooler (CAC) to cool compressed air from the turbocharger, before it enters the engine. Ambient air from outside the vehicle travels across the CAC to cool intake air passing through the inside of the CAC. Condensate may form in the CAC when the ambient air temperature decreases, or during humid or rainy weather conditions, where the intake air is cooled below the water dew point. When the intake air includes recirculated exhaust gasses, the condensate can become acidic and corrode the CAC housing. The corrosion can lead to leaks between the air charge, the atmosphere, and possibly the coolant in the case of water-to-air coolers. Condensate may accumulate in the CAC, and then be drawn into the engine at once during times of increased air mass flow, increasing the chance of engine misfire. Some CACs may have a self-cleanse mode which is activated during threshold levels of air mass flow that do not cause misfire. However, if the vehicle is not driven such that it reaches this threshold level, condensate may accumulate and then cause misfire by being drawn into the engine too quickly when downshifting from a higher to a lower transmission gear at wide open throttle. Air mass flow may increase to higher levels, increasing the chance of engine misfire, during multiple gear downshifts.
Other attempts to address engine misfire due to condensate ingestion involve avoiding condensate build-up. However, the inventors herein have recognized potential issues with such methods. Specifically, while some methods may reduce or slow condensate formation in the CAC, condensate may still build up over time. If this build-up cannot be stopped, ingestion of the condensate during downshifting, specifically during downshifts that skip one or more intermediate gears, may increase the chance of engine misfire.
In one example, the issues described above may be addressed by a method for performing a multiple gear downshift in stages, controlling the increase in air mass flow and condensate purging from the CAC. Specifically, a transmission gear may be downshifted from a higher gear to a lower gear by transiently operating in an intermediate gear before shifting to the lower gear. In this way, condensate may be purged from the CAC at a lower air mass flow, while in the intermediate gear. Thus, when finally downshifting to the lower gear, engine misfire may not occur due to increased air mass flow.
As one example, in response to a multiple gear downshift request, a transmission gear may be downshifted from a higher gear to a lower gear. If the requested downshift increases air mass flow above a threshold level, engine misfire may occur if the amount of boost or amount condensate in the CAC have reached threshold levels. The CAC may self-cleanse (purge condensate) when air mass flow reaches a threshold range. However, if air mass flow remains below the threshold range for a threshold duration, condensate may accumulate in the CAC and the threshold boost level and/or threshold level of condensate may be reached. Once these conditions have been reached, misfire may be reduced by controlling the execution of a requested multiple gear downshift. For example, in response to a multiple gear downshift request that will increase air mass flow over a threshold level, when air mass flow is less than a threshold range for a threshold duration, the transmission gear may be downshifted from a higher gear to an intermediate gear, and then to the requested lower gear. By holding the transmission gear at the intermediate gear for a duration, condensate may be blown off the CAC and into the engine at a slower rate. Then, when shifting to the lower gear, the increase in air mass flow may not cause engine misfire since stored condensate has already been purged from the CAC. In this way, engine misfire may be reduced during multiple gear downshifts by utilizing an intermediate gear to control the increase in air mass flow and resulting condensate purging from the CAC.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.